It is known to use a beam of electromagnetic radiation (typically from a laser) during simulated operation of a weapon for training purposes. In one type of system (U.K. patent specifications Nos. 1,228,143; 1,228,144; 1,439,612 and 1,451,192), the beam of radiation is pointed in the same direction as the weapon (for example, a gun) at the time of `firing` the ammunition (a shell or bullet) with adjustment for such factors as aim-off if appropriate. In another type (U.K. patent specifications Nos. 1,300,941 and 1,300,942) the beam is pointed to intersect continuously the path that the ammunition (for example, a missile) would follow in a live firing. In either case, the result is that the beam of radiation is directed at the point in space occupied by the ammunition when it reaches the vicinity of the target.
Such systems basically involve a device, commonly known as a projector, for generating, and if necessary orienting, the beam of radiation, and another device, known as a detector, for detecting incidence of the radiation on the target. The detector may be mounted on the target itself, or it may be associated with the projector, the radiation being reflected from the target by a retro-reflector mounted thereon.
In known systems, the projector has been arranged to generate radiation in the form of pulses of very short duration and relatively high peak power. Consequently, the detector (a photo-cell coupled to an amplifier) has been designed essentially to detect each pulse of radiation as an individual, discrete entity. Because of the abrupt nature of the pulses, the bandwidth of the detector amplifier has to be relatively large to ensure reliable detection of a pulse, which in turn limits to one the number of photo-cells which can be connected to an amplifier if an acceptable signal-to-noise ratio is to be maintained. In practice, a target needs to be fitted with at least four photo-cells to ensure detection of radiation from any direction around the target, and each of these photo-cells requires its own sensitive, stable, wide-bandwidth (and therefore expensive) amplifier.